Devices and methods for monitoring the environment with the help of echo signals are used in particular in motor vehicles, where obstacles in the environment of the vehicle are detected with the help of transceiver units, in particular for ultrasound, radar, and infrared signals. A plurality of such transceiver units may be fixedly mounted on the vehicle body, and if necessary a warning signal is generated to warn the driver of the vehicle of a potentially hazardous approach to an obstacle in complex traffic situations such as parking.
Such a monitoring device and a method for operating same are referred to in DE 40 23 538 A1, for example.
The accuracy with which such devices function depends to a great extent on the accuracy with which the relative positions of the plurality of transceiver units of such a monitoring device are known in relation to one another and can be taken into account in calculating the distance from an obstacle on the basis of the received echo signals.
In particular, a rough classification of the obstacle (wall, point, inside corner, . . . ) according to the mathematical equations referred to in DE 40 23 538 A1 requires accurate knowledge of the relative positions. In addition, in the case of an obstacle at a great distance, i.e., an obstacle whose distance from the vehicle is much greater than the distances between the transceiver units, the direct echoes detected by the individual transceiver units have propagation times that differ only slightly, and these propagation times constitute a usable approximate value for the actual distance, but in the case of a near point obstacle such as a post whose distance from the vehicle is less than the distance between the transceiver units, neither a direct echo nor a cross-echo is a usable approximation for the actual distance. For example, if the vehicle is approaching a post so that it could strike the post at a point between two transceiver units, then with progressive approach to the post, the propagation time of the cross-echo reflected back from it does not approach zero but instead approaches the propagation time corresponding to the distance between the two transceiver units. The accuracy with which this latter distance is known is therefore crucial for the lower limit up to which the monitoring device is still capable of estimating a distance from an obstacle with a usable accuracy.
This problem has resulted in echo-signal monitoring devices for motor vehicles being installed so far practically only in new vehicles by vehicle manufacturers. In retrofitting a vehicle with such devices, it may be very difficult to ensure the accuracy required for reliable measurements in positioning the transceiver units. This may be the case in particular when the retrofitting is performed by the vehicle owner himself.